A 70-year-old, 42kg female with chronic renal failure, Type 2 diabetes and a history of alcohol abuse was admitted for management of leg ulcers infected with MSSA. Ten days into her admission she became increasingly short of breath and was referred to ICU.

Parameter Patient value    Normal range
Sodium                                      139 mmol/l 134 – 146
Potassium                        4.4  mmol/l        3.4 – 5.0
Chloride                  115 mmol/l           100 – 110
Urea                           15.3* mmol/l 3.0 – 8.0
Creatinine                 309* umol/l 50 – 120
Glucose                     5.1 mmol/l 3.0 – 5.4
pH                               7.11* 7.35 – 7.45
PCO2                                    13* mmHg (1.7* kPa) 35 – 45 (4.6 – 6.0)
HCO3                                   4* mmol/l 22 – 27
Base excess              -24*                             -2 – +2
Measured osmolality  300 mOsm/kg           280 – 300

a) Describe this acid-base picture

b) Give three possible causes

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College Answer

a)
Severe compensated metabolic acidosis with a raised anion gap (~20), normal osmolar gap and delta ratio 0.4 (Some candidates might say that there is both a high AG and normal AG acidosis rather than stating the delta ratio and that is also correct).

b)
Renal failure

Pyroglutamic acidosis (renal and liver dysfunction and possible flucloxacillin and paracetamol exposure) 

Sepsis

Metformin related lactic acidosis
(delta ratio suggests mixed AG and NAG MA or renal failure)

Discussion

This is another one of these "interpret an ABG" questions.

How did they arrive at these answers?

Normal or slightly raised osmolar gap     

= 300 - ((1.86 x 139) + 15.4 + 5.1 + 9) = 12.

High anion gap                                            
The anion gap is (139) - (115 + 4) = 20, or 24.4 when calculated with potassium
 

Delta ratio = 0.4    
 The delta ratio, assuming a normal anion gap is 12 and a normal bicarbonate is 24, is
(20 - 12) / (24 - 4) = 0.4                                      
 

Thus, this is a high anion gap metabolic acidosis with hyperchloraemia, with normal osmolar gap, and incomplete respiratory compensation.

The MSSA mentioned in question 3.1 is a hint at flucloxacillin exposure, to make you think of pyroglutamic acidosis. Having never heard of pyroglutamic acidosis, I have found this article to explain it to myself. There is also another article here. A local summary of pyroglutamic acidosis is also available.

Pyroglutamic acidosis occurs due to glutathione depletion in patients who receive flucloxacillin or vigabatrin  together with paracetamol. The key feature is loss of feedback inhibition because of glutathione depletion, which results in overproduction of pyroglutamic acid.

References

References

Wrenn K. The delta gap: an approach to mixed acid-base disorders. Ann Emerg Med 1990 Nov; 19(11) 1310-3.

Dempsey GA Lyall HJ, Corke CF, Scheinkestel CD. Pyroglutamic acidemia: a cause of high anion gap metabolic acidosis. Crit Care Med. 2000Jun;28(6):1803-7.